The devil comes to Moscow. Chaos ensues. There's a bit of a love story, but mostly a long cast of characters (who are hard to keep straight because they're all Russian and so have five or six nicknames) meets an assortment of bad ends at the hands of the devil's wiley cohorts. It's a rowdy book, written during Stalin's Soviet regime (here are some biographical notes on Bulgakov), and suppressed until 1966, thirty years after Bulgakov's death, when his wife released it for publication.
Each character is trapped by his own vice (by greed, lust, etc.), and none of the characters come off looking particularly well, though I certainly liked some of them better than others--specifcally Koroviev and Behemoth. There's also a very interesting bit of reworking the gospel that occurs as Pontius Pilate makes an occasional appearance. Tampering with the gospel nearly always wins points with me, so long as it's done in a "tampering to examine the possibilities" sort of way, as opposed to "tampering to fit an agenda" (my favorite example of the first sort of tampering is Nikos Kazantzakis' The Last Temptation of Christ).
A fun book, chock full of devious behavior, social criticism and the brilliant line "Manuscripts don't burn." Reminded me a bit of Neil Gaiman & Terry Pratchett's Good Omens, but better, because it's Russian.
(Here is an interesting and related link.)